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Enolase is an enzyme that catalyzes a reaction of glycolysis. Glycolysis converts glucose into two 3-carbon molecules called pyruvate. The energy released during glycolysis is used to make ATP. Enolase is used to convert 2-phosphoglycerate (2PG) to phosphoenolpyruvate (PEP) in the 9th reaction of glycolysis: it is a reversible dehydration reaction. . Enolase is expressed abundantly in most cells and has been proven useful as a model to study mechanisms of enzyme action and structural analysis.

Structure
The secondary structure of enolase contains both alpha helices and beta sheets. The beta sheets are mainly parallel. As shown in the figure, enolase has about 36 alpha helices and 22 beta sheets (18 alpha helices and 11 beta sheets per domain). Enolase consists of two domains.

Structural Clasification of Proteins (SCOP) 

Enolase is in the alpha and beta proteins class and has a fold of TIM beta/alpha-barrel. It comes from the Superfamily on Enolase C-terminal domain-like and is in the enolase family.

Mechanism
The active site of enolase as shown, involves Lys 345, Lys 396, Glu 168, Glu 211, and His 159. Enolase forms a complex with two Mg 2+'s at its active site. The substrate, 2PG, binds to the two Mg2+'s, Glu 211, and Lys 345. The Mg 2+ then forms a bond at the deprotonated carboxylic acid on the 1'C to connect it with enolase. It also is connects to Glu 211 and Lys 345. Glu 211 makes a hydrogen bond with the alcohol group on the 3'C. Lys 345 deprotonates the 2'C and then the 2'C forms an alkene with the 1'C which then moves the electrons forming the ketone onto the oxygen making it have a negative charge. The other oxygen, which already has a negative charge, then moves its electron to form a ketone with the 1'C. The electrons that made up the alkene between the 1'C adn 2'C then moves to form an alkene between the 2'C and 3'C.  This breaks the bond with the alcohol on the 3'C which deprotonates Glu 211 on enolase to form H2O. Then the new molecule is released from enolase as PEP. PEP then goes on through another step in glycolysis to create pyruvate.

Fluoride ions inhibits glycolysis by forming a bond with Mg 2+ thus blocks the substrate (2PG) from binding to the active site of enolase.

Kinetics
Since Mg2+ is essential for binding the substrate, 2-PG, it is also needed at a specific quality in order to have a good rate, or velocity. The graph shows the V vs. [PGA], in which PGA is 2-PG, with two different concentrations of Mg2+. The upper curve, which also has greater Vmax, has an Mg2+ concentration of 10^-3 M while the lower curve, which has a lower Vmax, has an Mg2+ concentration of 10^-2 M. The Km is also larger the upper curve making the higher [Mg2+] more desirable.

Regulation
Enolase is found on the surface of a variety of eukaryotic cells as a strong plamingoen-binding receptor and on the surface of hematopietic celss such as monocytes, T cells and B cells, neuronal celss and endothelial cells. Enolase in muscle can bind other glycolytic enzymes, such as phosphoglycerate mutase, muscle creatine kinase, pyruvate kinase, and muscle troponin, with high affinity. This suggests that they make a functional glycolytic segment in the muscle where ATP production is required in order for the muscle to contract. Myc-binding protein (MBP-1) is similar to the a-enolse structure and is found in the nucleus as a DNA-binding protein. Enolase is regulated by the concentration of Mg2+ and the previous steps of glycolysis.

Additional Resources
For additional information, see: Carbohydrate Metabolism